Method and System for Controlling Electrical Conditions of Tissue II

ABSTRACT

A method for controlling electrical conditions of tissue in relation to a current stimulus. A first current produced by a first current source is delivered to the tissue via a current injection electrode. A second current drawn by a second current source is extracted from the tissue via a current extraction electrode. The second current source is matched with the first current source so as to balance the first current and the second current. A ground electrode which is proximal to the current injection electrode and the current extraction electrode is grounded, to provide a ground path for any mismatch current between the first current and second current. A response of the tissue to the current stimulus is measured via at least one measurement electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Australian Provisional Patent Application No. 2012904838 filed 6 Nov. 2012, which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to controlling the electrical conditions of tissue, for example for use in suppressing artefact to enable improved measurement of a response to a stimulus, such as measurement of a compound action potential by using one or more electrodes implanted proximal to a neural pathway.

BACKGROUND OF THE INVENTION

Neuromodulation is used to treat a variety of disorders including chronic pain, Parkinson's disease, and migraine. A neuromodulation system applies an electrical pulse to tissue in order to generate a therapeutic effect. When used to relieve chronic pain, the electrical pulse is applied to the dorsal column (DC) of the spinal cord or dorsal root ganglion (DRG). Such a system typically comprises an implanted electrical pulse generator, and a power source such as a battery that may be rechargeable by transcutaneous inductive transfer. An electrode array is connected to the pulse generator, and is positioned in the dorsal epidural space above the dorsal column. An electrical pulse applied to the dorsal column by an electrode causes the depolarisation of neurons, and generation of propagating action potentials. The fibres being stimulated in this way inhibit the transmission of pain from that segment in the spinal cord to the brain.

While the clinical effect of spinal cord stimulation (SCS) is well established, the precise mechanisms involved are poorly understood. The DC is the target of the electrical stimulation, as it contains the afferent Aβ fibres of interest. Aβ fibres mediate sensations of touch, vibration and pressure from the skin. The prevailing view is that SCS stimulates only a small number of Aβ fibres in the DC. The pain relief mechanisms of SCS are thought to include evoked antidromic activity of Aβ fibres having an inhibitory effect, and evoked orthodromic activity of Aβ fibres playing a role in pain suppression. It is also thought that SCS recruits Aβ nerve fibres primarily in the DC, with antidromic propagation of the evoked response from the DC into the dorsal horn thought to synapse to wide dynamic range neurons in an inhibitory manner.

Neuromodulation may also be used to stimulate efferent fibres, for example to induce motor functions. In general, the electrical stimulus generated in a neuromodulation system triggers a neural action potential which then has either an inhibitory or excitatory effect. Inhibitory effects can be used to modulate an undesired process such as the transmission of pain, or to cause a desired effect such as the contraction of a muscle.

The action potentials generated among a large number of fibres sum to form a compound action potential (CAP). The CAP is the sum of responses from a large number of single fibre action potentials. The CAP recorded is the result of a large number of different fibres depolarising. The propagation velocity is determined largely by the fibre diameter and for large myelinated fibres as found in the dorsal root entry zone (DREZ) and nearby dorsal column the velocity can be over 60 ms⁻¹. The CAP generated from the firing of a group of similar fibres is measured as a positive peak potential P1, then a negative peak N1, followed by a second positive peak P2. This is caused by the region of activation passing the recording electrode as the action potentials propagate along the individual fibres.

To better understand the effects of neuromodulation and/or other neural stimuli, it is desirable to record a CAP resulting from the stimulus. However, this can be a difficult task as an observed CAP signal will typically have a maximum amplitude in the range of microvolts, whereas a stimulus applied to evoke the CAP is typically several volts. Electrode artefact usually results from the stimulus, and manifests as a decaying output of several millivolts throughout the time that the CAP occurs, presenting a significant obstacle to isolating the CAP of interest. Some neuromodulators use monophasic pulses and have capacitors to ensure there is no DC flow to the tissue. In such a design, current flows through the electrodes at all times, either stimulation current or equilibration current, hindering spinal cord potential (SCP) measurement attempts. The capacitor recovers charge at the highest rate immediately after the stimulus, undesirably causing greatest artefact at the same time that the evoked response occurs.

To resolve a 10 uV SCP with 1 uV resolution in the presence of an input 5V stimulus, for example, requires an amplifier with a dynamic range of 134 dB, which is impractical in implant systems. As the neural response can be contemporaneous with the stimulus and/or the stimulus artefact, CAP measurements present a difficult challenge of amplifier design. In practice, many non-ideal aspects of a circuit lead to artefact, and as these mostly have a decaying exponential appearance that can be of positive or negative polarity, their identification and elimination can be laborious.

A number of approaches have been proposed for recording a CAP. King (U.S. Pat. No. 5,913,882) measures the spinal cord potential (SCP) using electrodes which are physically spaced apart from the stimulus site. To avoid amplifier saturation during the stimulus artefact period, recording starts at least 1-2.5 ms after the stimulus. At typical neural conduction velocities, this requires that the measurement electrodes be spaced around 10 cm or more away from the stimulus site, which is undesirable as the measurement then necessarily occurs in a different spinal segment and may be of reduced amplitude.

Nygard (U.S. Pat. No. 5,785,651) measures the evoked CAP upon an auditory nerve in the cochlea, and aims to deal with artefacts by a sequence which comprises: (1) equilibrating electrodes by short circuiting stimulus electrodes and a sense electrode to each other; (2) applying a stimulus via the stimulus electrodes, with the sense electrode being open circuited from both the stimulus electrodes and from the measurement circuitry; (3) a delay, in which the stimulus electrodes are switched to open circuit and the sense electrode remains open circuited; and (4) measuring, by switching the sense electrode into the measurement circuitry. Nygard also teaches a method of nulling the amplifier following the stimulus. This sets a bias point for the amplifier during the period following stimulus, when the electrode is not in equilibrium. As the bias point is reset each cycle, it is susceptible to noise. The Nygard measurement amplifier is a differentiator during the nulling phase which makes it susceptible to pickup from noise and input transients when a non-ideal amplifier with finite gain and bandwidth is used for implementation.

Daly (US Patent Application No. 2007/0225767) utilizes a biphasic stimulus plus a third phase “compensatory” stimulus which is refined via feedback to counter stimulus artefact. As for Nygard, Daly's focus is the cochlea. Daly's measurement sequence comprises (1) a quiescent phase where stimulus and sense electrodes are switched to Vdd; (2) applying the stimulus and then the compensatory phase, while the sense electrodes are open circuited from both the stimulus electrodes and from the measurement circuitry; (3) a load settling phase of about 1 μs in which the stimulus electrodes and sense electrodes are shorted to Vdd; and (4) measurement, with stimulus electrodes open circuited from Vdd and from the current source, and with sense electrodes switched to the measurement circuitry. However a 1 μs load settling period is too short for equilibration of electrodes which typically have a time constant of around 100 μs. Further, connecting the sense electrodes to Vdd pushes charge onto the sense electrodes, exacerbating the very problem the circuit is designed to address.

Evoked responses are less difficult to detect when they appear later in time than the artefact, or when the signal-to-noise ratio is sufficiently high. The artefact is often restricted to a time of 1-2 ms after the stimulus and so, provided the neural response is detected after this time window, data can be obtained. This is the case in surgical monitoring where there are large distances between the stimulating and recording electrodes so that the propagation time from the stimulus site to the recording electrodes exceeds 2 ms.

Because of the unique anatomy and tighter coupling in the cochlea, cochlear implants use small stimulation currents relative to the tens of mA sometimes required for SCS, and thus measured signals in cochlear systems present a relatively lower artefact. To characterize the responses from the dorsal columns, high stimulation currents and close proximity between electrodes are required. Moreover, when using closely spaced electrodes both for stimulus and for measurement the measurement process must overcome artefact directly, in contrast to existing “surgical monitoring” techniques involving measurement electrode(s) which are relatively distant from the stimulus electrode(s).

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

In this specification, a statement that an element may be “at least one of” a list of options is to be understood that the element may be any one of the listed options, or may be any combination of two or more of the listed options.

SUMMARY OF THE INVENTION

According to a first aspect the present invention provides a method for controlling electrical conditions of tissue in relation to a current stimulus, the method comprising:

delivering to the tissue via a current injection electrode a first current produced by a first current source;

extracting from the tissue via a current extraction electrode a second current drawn by a second current source, the second current source being matched with the first current source so as to balance the first current and the second current;

grounding a ground electrode which is proximal to the current injection electrode and the current extraction electrode, to provide a ground path for any mismatch current between the first current and second current; and

measuring via at least one measurement electrode a response of the tissue to the current stimulus.

According to a second aspect the present invention provides an implantable device for controlling electrical conditions of tissue in relation to a current stimulus, the device comprising:

a plurality of electrodes including at least one nominal current injection electrode, at least one nominal current extraction electrode, at least one nominal ground electrode which is proximal to the current injection electrode and the current extraction electrode, and at least one nominal measurement electrode, the electrodes being configured to be positioned proximal to the tissue to make electrical contact with the tissue;

a first current source for producing a first current to be delivered to the tissue by the current injection electrode;

a second current source for producing a second current to be extracted from the tissue via the current extraction electrode, the second current source being matched with the first current source so as to balance the first current and the second current;

an electrical ground for grounding the ground electrode, to provide a ground path for any mismatch current between the first current and second current; and

measurement circuitry for measuring via the at least one measurement electrode a response of the tissue to the current stimulus.

In preferred embodiments of the invention the ground electrode is connected to ground throughout application of a stimulus by the first and second current sources. Alternatively, in some embodiments of the invention the ground electrode may be disconnected, or floating, during some or all of the application of the stimulus.

In preferred embodiments, the ground electrode and the measurement electrode are located outside the dipole formed by the current injection electrode and the current extraction electrode. In such embodiments the operation of the ground electrode acts to spatially shield the measurement electrode from the stimulus field, noting that the voltage at points between the poles of a dipole is comparable to the voltage on the electrodes, whereas outside the dipole the voltage drops with the square of distance.

Preferred embodiments of the invention may thus reduce artefact by reducing interaction between the stimulus and the measurement recording via a measurement amplifier input capacitance.

Some embodiments of the invention may utilise a blanking circuit for blanking the measurement amplifier during and/or close in time to the application of a stimulus. However, alternative embodiments may utilise an unblanked measurement amplifier, which connects a measurement electrode to an analog-to-digital circuit, significantly reducing complexity in the measurement signal chain.

The electrical ground may be referenced to a patient ground electrode distal from the array such as a device body electrode, or to a device ground. Driving the ground electrode to electrical ground will thus act to counteract any non-zero stimulus artefact produced by mismatched currents during application of the stimulus.

The electrodes are preferably part of a single electrode array, and are physically substantially identical whereby any electrode of the array may serve as any one of the nominal electrodes at a given time. Alternatively the electrodes may be separately formed, and not in a single array, while being individually positioned proximal to the tissue of interest.

In preferred embodiments of the invention, the ground electrode, current injection electrode, current extraction electrode and measurement electrode are selected from an implanted electrode array. The electrode array may for example comprise a linear array of electrodes arranged in a single column along the array. Alternatively the electrode array may comprise a two dimensional array having two or more columns of electrodes arranged along the array. Preferably, each electrode of the electrode array is provided with an associated measurement amplifier, to avoid the need to switch the sense electrode(s) to a shared measurement amplifier, as such switching can add to measurement artefact. Providing a dedicated measurement amplifier for each sense electrode is further advantageous in permitting recordings to be obtained from multiple sense electrodes simultaneously.

In the first and second aspects of the invention, the measurement may be a single-ended measurement obtained by passing a signal from a single sense electrode to a single-ended amplifier. Alternatively, the measurement may be a differential measurement obtained by passing signals from two measurement electrodes to a differential amplifier. In some embodiments three stimulus electrodes may be used to apply a tripolar stimulus for example by using one current injection electrode and two current extraction electrodes driven by respective extraction current sources which together are balanced to the injection current source. The stimulus may be monophasic, biphasic, or otherwise.

Embodiments of the invention may prove beneficial in obtaining a CAP measurement which has lower dynamic range and simpler morphology as compared to systems more susceptible to artefact. Such embodiments of the present invention may thus reduce the dynamic range requirements of implanted amplifiers, and may avoid or reduce the complexity of signal processing systems for feature extraction, simplifying and miniaturizing an implanted integrated circuit. Such embodiments may thus be particularly applicable for an automated implanted evoked response feedback system for stimulus control.

According to another aspect the present invention provides a computer program product comprising computer program code means to make an implanted processor execute a procedure for controlling electrical conditions of neural tissue, the computer program product comprising computer program code means for carrying out the method of the first aspect.

According to a further aspect the present invention provides a computer readable storage medium, excluding signals, loaded with computer program code means to make an implanted processor execute a procedure for controlling electrical conditions of neural tissue, the computer readable storage medium loaded with computer program code means for carrying out the method of the first aspect.

The present invention recognises that when considering spinal cord stimulation, obtaining information about the activity within the spinal segment where stimulation is occurring is highly desirable. Observing the activity and extent of propagation both above (rostrally of) and below (caudally of) the level of stimulation is also highly desirable. The present invention recognises that in order to record the evoked activity within the same spinal segment as the stimulus requires an evoked potential recording system which is capable of recording an SCP within approximately 3 cm of its source, i.e. within approximately 0.3 ms of the stimulus, and further recognises that in order to record the evoked activity using the same electrode array as applied the stimulus requires an evoked potential recording system which is capable of recording an SCP within approximately 7 cm of its source, i.e. within approximately 0.7 ms of the stimulus.

In some embodiments the method of the present invention may be applied to measurement of other bioelectrical signals, such as muscle potentials. The method of the present invention may be applicable to any measurement of any voltage within tissue during or after stimulation, and where the stimulation may obscure the voltage being measured. Such situations include the measurement of evoked spinal cord potentials, potentials evoked local to an electrode during deep brain stimulation (DBS), the measurement of EEGs during deep brain stimulation (where the source of the potential is distant from the stimulating electrodes), the measurement of signals in the heart (ECGs) by a pacemaker, the measurement of voltages in stimulated muscles (EMGs), and the measurement of EMGs triggered by the stimulation of distant and controlling nervous tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates an implantable device suitable for implementing the present invention;

FIG. 2 illustrates currents and voltages which can contribute to SCP measurements;

FIG. 3 illustrates the equivalent circuit of a typical system for applying a neural stimulus and attempting to measure a neural response;

FIG. 4 is an equivalent circuit modelling the tissue/electrode interface and electrode loading;

FIG. 5 illustrates a circuit having the problem of mismatched current sources;

FIG. 6 illustrates another embodiment of the present invention;

FIGS. 7 a and 7 b plot the electrode voltages arising during stimulation in the circuits of FIGS. 3 and 6 respectively, while FIGS. 7 c and 7 d respectively plot the artefact on the sense electrodes during such stimuli; and

FIG. 8 a plots the measurements from an electrode array in response to a stimulus delivered by the array to a sheep dorsal column, while FIG. 8 b is a superimposed plot of similar data, demonstrating timing of respective signal features.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an implantable device 100 suitable for implementing the present invention. Device 100 comprises an implanted control unit 110, which controls application of a sequence of neural stimuli. In this embodiment the unit 110 is also configured to control a measurement process for obtaining a measurement of a neural response evoked by a single stimulus delivered by one or more of the electrodes 122. Device 100 further comprises an electrode array 120 consisting of a three by eight array of electrodes 122, each of which may be selectively used as the stimulus electrode, sense electrode, compensation electrode or sense electrode.

FIG. 2 shows the currents and voltages that contribute to spinal cord potential (SCP) measurements in a typical system of the type shown in FIG. 3. These signals include the stimulus current 202 applied by two stimulus electrodes, which is a charge-balanced biphasic pulse to avoid net charge transfer to or from the tissue and to provide low artefact. Alternative embodiments may instead use three electrodes to apply a tripolar charge balanced stimulus for example where a central electrode. In the case of spinal cord stimulation, the stimulus currents 202 used to provide paraesthesia and pain relief typically consist of pulses in the range of 3-30 mA amplitude, with pulse width typically in the range of 100-400 μs, or alternatively may be paraesthesia-free such as neuro or escalator style stimuli. The stimuli can comprise monophasic or biphasic pulses.

The stimulus 202 induces a voltage on adjacent electrodes, referred to as stimulus crosstalk 204. Where the stimuli 202 are SCP stimuli they typically induce a voltage 204 in the range of about 1-5 V on a SCP sense electrode.

The stimulus 202 also induces electrode artefact. The mechanism of artefact production can be considered as follows. The stimulus crosstalk can be modelled as a voltage, with an equivalent output impedance. In a human spinal cord, this impedance is typically around 500 ohms per electrode, but will be larger or smaller in different applications. This resistance has little effect in the circuit, but is included for completeness. The stimulus crosstalk drives the measurement amplifiers through the electrode/tissue interface. This interface is shown in FIG. 4 as a set of series capacitance/resistance pairs, modelling a component referred to in the literature as a “Warburg element”. The RC pairs model the complex diffusion behaviour at the electrode surface, and have time constants from micro-seconds to seconds. The cables from the electrode to the amplifier add capacitance which loads the electrode, along with the resistive input impedance of the amplifier itself. Typical loading would be 200 pF of capacitance and 1 megohms of resistance. Following this is an ideal amplifier in this equivalent circuit of FIG. 4.

The electrode artefact is the response of the electrode/tissue interface, when driven by the stimulus crosstalk and loaded by the capacitance and resistance at the amplifier input. It can be observed, either with a circuit simulator or in a laboratory. It can also be observed that the sign of the artefact is opposite for capacitive and resistive loading. Electrical artefact usually also arises from the behaviour of the amplifier circuitry in response to these particular circumstances.

It is possible to reduce artefact by reducing the loading on the electrode, however in practical situations there are limits to how low this capacitance can be made. Increasing the electrode surface area also decreases artefact but again in practical situations there will be limits to the electrode size. Artefact can also be reduced by adding resistance or capacitance to the amplifier input relying on the opposite sign of the artefact produced by these terms. However, this only works to a limited extent, and changing the size of the electrode changes the size of the required compensation components which makes it difficult to make a general purpose amplifier that can be connected to a range of electrodes. One can also reduce artefact by reducing the size of the stimulus crosstalk, and this is the aim of the embodiment of this invention shown in FIG. 6, which relates to evoking and measuring a neural response.

Referring again to FIGS. 2 and 3, an appropriate electrical stimulus 202 will induce nerves to fire, and thereby produces an evoked neural response 206. In the spinal cord, the neural response 206 can have two major components: a fast response lasting ˜2 ms and a slow response lasting ˜15 ms. The slow response only appears at stimulation amplitudes which are larger than the minimum stimulus required to elicit a fast response. Many therapeutic stimuli paradigms seek to evoke fast responses only, and to avoid evoking any slow response. Thus, the neural response of interest for neural response measurements concludes within about 2 ms. The amplitude of the evoked response seen by epidural electrodes is typically no more than hundreds of microvolts, but in some clinical situations can be only tens of microvolts.

In practical implementation a measurement amplifier used to measure the evoked response does not have infinite bandwidth, and will normally have infinite impulse response filter poles, and so the stimulus crosstalk 204 will produce an output 208 during the evoked response 206, this output being referred to as electrical artefact.

Electrical artefact can be in the hundreds of millivolts as compared to a SCP of interest in the tens of microvolts. Electrical artefact can however be somewhat reduced by suitable choice of a high-pass filter pole frequency.

The measurement amplifier output 210 will therefore contain the sum of these various contributions 202-208. Separating the evoked response of interest (206) from the artefacts 204 and 208 is a significant technical challenge. For example, to resolve a 10 μV SCP with 1 μV resolution, and have at the input a 5V stimulus, requires an amplifier with a dynamic range of 134 dB. As the response can overlap the stimulus this represents a difficult challenge of amplifier design.

FIG. 5 illustrates a problem of mismatched current sources, and FIG. 6 illustrates an embodiment in accordance with the present invention. In FIG. 5, a first current source injects a current stimulus (+I) to the tissue via an injection electrode. A second current source extracts an extraction current (−I) via an extraction electrode. However, some slight mismatch between the first and second current sources is inevitable, so that a mismatch current (dI) will leak via stray impedances Z, giving rise to some unknown mismatch voltage in the tissue, corrupting measurements of evoked responses. Since the current into the amplifier output exactly matches the current from the current source, one could consider using two matched current sources. However, with non-ideal sources the current sources do not match. We call the error in the current match “dI”. The mismatch is driven into the impedance from bulk tissue to ground Z. This is usually large, so the electrodes are exposed to a large voltage dI.Z. This voltage can be close to the full supply voltage—if (say) the positive current source outputs more current than the negative source, the tissue will be driven positive until the positive current source saturates, and the current between the two sources is exactly balanced.

In contrast, FIG. 6 illustrates an embodiment in accordance with the present invention, in which an error sink electrode, or ground electrode, is provided and is interposed between the stimulus electrodes and the measurement electrodes. Thus, by adding an additional electrode connected to ground, this mismatch current has a place to go. The voltage on the bulk tissue is dI.R, the current source mismatch multiplied by the tissue impedance R. This will be small relative to dI.Z. This therefore reduces the electrode crosstalk to a small value. In alternative embodiments, the error sink electrode could be driven by “active ground” circuitry which uses feedback to seek to drive the tissue electrical conditions to ground. A suitable active ground circuit concept is disclosed in Australian provisional patent application no. 2012904836 entitled “Method and System for Controlling Electrical Conditions of Tissue”, by the present applicant.

The plots of FIG. 7 show the electrode voltages in a 100 ohm star load at 5 mA stimulus current and 360 us interphase gap. Trace 712 is from the stimulus electrode and trace 714 is from the ground electrode, while traces 716 and 718 are from two nominal sense electrodes, respectively. In FIG. 7 a the stimulation configuration of FIG. 3 was used, namely a stimulating electrode was driven by a current source and a nearby electrode was grounded to provide a path for current flow. The biphasic stimulus evident in trace 712 was applied to a 1/10 PBS saline solution. As can be seen in traces 716 and 718 considerable crosstalk artefact arises on the sense electrodes when using such a stimulus configuration.

In contrast to FIG. 7 a, FIG. 7 b shows the result when matched current sources and a ground electrode are used, in accordance with one embodiment of the present invention. In FIG. 7 b, the same biphasic stimulus is applied via a first stimulus electrode to give rise to trace 722 on that electrode, while the matched negative current source gives rise to voltage 724 on an adjacent second stimulus electrode. A third electrode near the current sources is grounded in accordance with the present invention (voltage trace not shown in FIG. 7 b). Traces 726 and 728 were obtained from two sense electrodes, and show that the stimulus crosstalk has been significantly reduced. These traces show that the technique of FIG. 6 produces low artefact in traces 726 and 728.

FIGS. 7 c and 7 d illustrate the artefact on the same two sense electrodes, denoted electrodes 4 (solid) and 5 (dashed), during normal stimulation as reflected in FIG. 7 a. FIG. 7 d shows the artefact on the same electrodes 4 and 5 during the stimulation reflected by FIG. 7 b. As can be seen, the artefact has been reduced from about 450 μV to about 100 μV by use of the present embodiment of the present invention.

FIG. 8 a shows the evoked response in a sheep dorsal column. In particular, FIG. 8 a plots the measurements obtained simultaneously from 22 electrodes of a 24 electrode array in response to a stimulus delivered by two adjacent electrodes positioned centrally in the array. As can be seen, evoked responses propagate simultaneously both caudally and rostrally from the central stimulus site. The current required to evoke such a response in a sheep is much lower than in humans, and the evoked response signals are higher, so artefact is less of a problem. In other regards the sheep signals are similar to the human case. In FIG. 8 a the amplifiers are unblanked at approximately 0.75 msec and the response finishes within another 0.75 ms. FIG. 8 b is a superimposed plot of similar data, demonstrating timing of respective signal features when measuring on multiple electrodes at increasing distance from the stimulus site. FIGS. 8 a and 8 b illustrate the importance of reducing artefact during the period immediately after stimulation.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. For example while application of the method to neural stimulation is described, it is to be appreciated that the techniques described in this patent apply in other situations involving measurement of a voltage within tissue during or after stimulation.

The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 

1. A method for controlling electrical conditions of tissue in relation to a current stimulus, the method comprising: delivering to the tissue via a current injection electrode a first current produced by a first current source; extracting from the tissue via a current extraction electrode a second current drawn by a second current source, the second current source being matched with the first current source so as to balance the first current and the second current; grounding a ground electrode which is proximal to the current injection electrode and the current extraction electrode, to provide a ground path for any mismatch current between the first current and second current; and measuring via at least one measurement electrode a response of the tissue to the current stimulus.
 2. The method of claim 1 wherein the ground electrode is connected to ground throughout application of a stimulus by the first and second current sources.
 3. The method of claim 1 wherein the ground electrode is disconnected, or floating, during some or all of the application of the stimulus.
 4. The method of claim 1 wherein the ground electrode and the measurement electrode are located outside a dipole formed by the current injection electrode and the current extraction electrode.
 5. The method of claim 1 wherein the ground electrode is grounded to a distal patient ground electrode.
 6. An implantable device for controlling electrical conditions of tissue in relation to a current stimulus, the device comprising: a plurality of electrodes including at least one nominal current injection electrode, at least one nominal current extraction electrode, at least one nominal ground electrode which is proximal to the current injection electrode and the current extraction electrode, and at least one nominal measurement electrode, the electrodes being configured to be positioned proximal to the tissue to make electrical contact with the tissue; a first current source for producing a first current to be delivered to the tissue by the current injection electrode; a second current source for producing a second current to be extracted from the tissue via the current extraction electrode, the second current source being matched with the first current source so as to balance the first current and the second current; an electrical ground for grounding the ground electrode, to provide a ground path for any mismatch current between the first current and second current; and measurement circuitry for measuring via the at least one measurement electrode a response of the tissue to the current stimulus.
 7. The implantable device of claim 6 wherein the ground electrode and the measurement electrode are located outside a dipole formed by the current injection electrode and the current extraction electrode.
 8. The implantable device of claim 6 wherein the ground electrode is grounded to a distal patient ground electrode. 